This invention relates to intraluminal endovascular stenting, a method by which a prosthesis is inserted into a body tube and expanded so as to reopen a collapsed vessel wall and prevent the wall from recollapsing into the lumen. Endovascular stenting is particularly useful for arteries which are blocked or narrowed and is an alternative to surgical procedures that intend to bypass the occlusion.
Percutaneous transluminal coronary angioplasty (PTCA) is used to open coronary arteries which have been occluded by a build-up of cholesterol fats or atherosclerotic plaque. Typically a guidewire is steered through the vascular system to the site of therapy. A guiding catheter, for example, can then be advanced over the guidewire and a balloon catheter advanced within the guiding catheter over the guidewire. The balloon at the distal end of the catheter is inflated causing the site of the stenosis to widen. The dilatation of the occlusion, however, can form flaps, fissures and dissections which threaten re-closure of the dilated vessel or even perforations in the vessel wall. Implantation of a metal stent can provide support for such flaps and dissections and thereby prevent reclosure of the vessel or provide a patch repair for a perforated vessel wall until corrective surgery can be performed. Reducing the possibility of restenosis after angioplasty reduces the likelihood that a secondary angioplasty procedure or a surgical bypass operation will be necessary.
An implanted prosthesis such as a stent can preclude additional procedures and maintain vascular patency by mechanically supporting dilated vessels to prevent vessel collapse. Stents can also be used to repair aneurysms, to support artificial vessels as liners of vessels or to repair dissections. Stents are suited to the treatment of any body lumen, including the vas deferens, ducts of the gallbladder, prostate gland, trachea, bronchus and liver. The body lumens range in size from 1.5 mm in the coronary vessels to 30 mm in the aortic vessel.
A stent typically is a cylindrically shaped device formed from wire(s)or a tube and intended to act as a permanent prosthesis. A stent is deployed in a body lumen from a radially compressed configuration into a radially expanded configuration which allows it to contact and support a body lumen. The stent can be made to be radially self-expanding or expandable by the use of an expansion device. The self expanding stent is made from a resilient springy material while the device expandable stent is made from a material which is plastically deformable. A plastically deformable stent can be implanted during an angioplasty procedure by using a balloon catheter bearing a stent which has been crimped onto the balloon. Stents radially expand as the balloon is inflated, forcing the stent into contact with the body lumen thereby forming a supporting relationship with the vessel walls. Deployment is effected after the stent has been introduced percutaneously, transported transluminally and positioned at a desired location by means of the balloon catheter.
A balloon of appropriate size and pressure is first used to open the lesion. The process is repeated with a stent crimped on a balloon. The stent is deployed when the balloon is inflated. The stent remains as a permanent scaffold after the balloon is withdrawn. A balloon capable of withstanding relatively high inflation pressures may be preferable for stent deployment because the stent must be forced against the artery""s interior wall so that it will fully expand thereby precluding the ends of the stent from hanging down into the channel encouraging the formation of thrombus.
Previous structures used as stents or intraluminal vascular grafts have included coiled stainless steel springs; helical wound spring coil made from shape memory alloy; expanding metal stents formed in a zig-zag pattern; diamond shaped, rectangular shaped, and other mesh and non-mesh designs. Exemplary stent devices are disclosed in U.S. Pat. No. 5,776,161 issued to Globerman, U.S. Pat. No. 5,449,373 issued to Pinchasik et al, U.S. Pat. No. 5,643,312 issued to Fischell et al and U.S. Pat. No. 5,421,955 issued to Lau et al.
Problems to be overcome in stent design include inadequate radial force to maintain expansion; inadequate scaffolding of tissue to the wall; pre-dilated longitudinal rigidity which negatively impacts on stent delivery; and shortening of the stent as a consequence of radial expansion. Predilation stent longitudinal rigidity is a significant shortcoming, and prevents the threading of the stent through long tortuous vessels and lesions. Shortening of the stent is also a problem, as it is important that the stent cover the entire lesion to minimize the risk of post-operative complications. Many of these problems are the result of difficult design problems resulting from the often conflicting goals of stent design. For example, it is desirable to have a high degree of scaffolding in the stent when the stent is expanded to its rated radial size so that the vessel wall will have uniform support. However, it is also desirable to have a small, relatively smooth delivered profile when the stent is mounted on the catheter to permit the stent and catheter to traverse small diameter lesions. The person skilled in the art will appreciate that as a stent with a very small delivered profile expands radially its structural elements become farther apart and create openings which reduce the amount of scaffolding available to support the vessel. A similar situation exists with respect to the conflicting goals of improved scaffolding and flexibility during catheter delivery since proper scaffolding will not be accomplished if there are few supporting structural elements and yet a stent with too many structural elements may be difficult to crimp onto the balloon catheter such that the structural elements will not abut or interfere with each other during delivery through tortuous vessels. Also, in some stents, during plastic deformation of the stent (i.e. balloon expansion) the strain is concentrated at small zones. This limits the properties of the material that can be used as well as the radial force and the expansion rate.
U.S. Pat. No. 5,776,161 issued to Globerman, which is incorporated by reference herein in its entirety, addresses a number of these issues. Globerman discloses an expandable stent having a small initial diameter, flexibility along its longitudinal axis prior to expansion and minimization of rigid local strain on the stent material by the presence of rotation joints which have minimal strain during stent expansion. The stent is substantially the same length before and after expansion and being flexible longitudinally when constrained, it is easy to deliver. However additional improvements in longitudinal flexibility in the crimped stent during delivery and scaffolding after delivery are still desired.
These and other objects are accomplished by the present invention. The medical device of the present invention includes a catheter and a stent mounted on the catheter, the stent having a hollow, cylindrical body made with a plurality of rings. The rings each extend circumferentially around the cylindrical body and include an undulating series of peaks and valleys. Typically, the undulating peaks and valleys of the rings are formed by opposing curved segments joined to each other by substantially straight segments. The rings are joined together by a series of links which are shaped and arranged to promote longitudinal flexibility as the stent is delivered on the catheter and effective scaffolding after deployment and to prevent shortening of the stent as the stent is expanded.
In one aspect of the invention, the rings are provided with inflection points on some portions of the rings which extend between an adjacent peak and valley of the ring. At each inflection point, a portion of the ring extends in a generally circumferential direction for a short distance. Typically, the inflection point is substantially centered between a peak and a valley of the ring. A link is joined at one end at the inflection point on one ring and also joined at a second end at a second inflection point on an adjacent ring. This link joins the rings together. Preferably, the link includes at least two curved segments in the unexpanded device which are capable of deflecting to promote the tendency of the stent to flex longitudinally when it is subjected to bending forces such as those encountered during delivery of the stent and catheter through a tortuous coronary artery. Also preferably, the short portion of the ring at the inflection point which extends generally circumferentially has a length measured circumferentially which is at least as great as the width of the link to which it is attached. Preferably, the circumferential length is no more than about twice the width of the link to which it is attached. This promotes the scaffolding provided to the vessel by the expanded stent since the links can be fit together closely in a nested arrangement with the undulations of the rings as the stent is crimped on the balloon catheter. By xe2x80x9cnestxe2x80x9d, xe2x80x9cnestedxe2x80x9d or nestingxe2x80x9d herein we mean that the elements are conformally arranged such they can be in very close proximity when the stent is crimped onto the catheter but without substantial contact that would affect the ability of the various elements to move in relation to each other as the stent and catheter are advanced through a tortuous body vessel. Where the undulations of the ring include generally straight segments between the peaks and valleys, the straight segments may be interrupted by an inflection point which produces a offset portion in the straight segment in a generally circumferential direction. In some preferred embodiments of the invention, no more than one link is connected to either of the first and second inflection points. This makes the inflection point a xe2x80x9cdead endxe2x80x9d in the longitudinal extent of the connecting links for the stent and permits some of the flexing forces which are not absorbed by the link itself to be absorbed by the rings to which it is attached. The links can be arranged to provide flexibility whether the peaks and valleys of the rings are arranged to make the rings appear to be mirror images to each other (i.e. peaks line up with or closely approach each other) or whether the peaks and valleys are paired with each other in an in-phase relationship or any alignment of the rings intermediate to those positions. Preferably, the rings are joined by multiple links (most preferably 3 or more) and have the same number of inflection points on each ring as the number of attaching links. When a large number of connecting links are employed, any curves or bends in the links are preferably of a complimentary shape to each other such that they will nest together when the stent is crimped onto the catheter.
Another aspect of the invention is the conformal nesting of ring and link components such that the stent can be readily crimped onto a balloon or other expansion device on the catheter. The stent made according to the present invention may be made from a tube which is cut with lasers or other techniques which are well known to those skilled in the art. The initial pattern cut into the tube includes link and ring components which cooperate with each other but which provide sufficient spacing between components that the stent can be crimped onto a catheter without causing general abutment of the ring and link components with each other and also permit longitudinal movement of the link components without disturbing the crimp of the ring components on the catheter during deployment of the stent through tortuous coronary arteries. The need for spacing between the components in the crimped condition must be balanced with the need to provide improved scaffolding of the vessel being treated. A relatively abundant number of links provides improved scaffolding of the vessel but potentially interferes with the ability to crimp the stent onto the catheter. In the present invention, the inflection points can provide the spacing needed for the nesting of the ring and link components by extending the ring in a generally circumferential direction for a distance which is sufficient to accommodate the width of the link component and provide space needed between the link components and the ring components which allows the stent to be crimped onto the catheter. In some embodiments of the invention, each inflection point includes an attachment to two connecting links extending in opposing directions and the circumferential offset at the inflection point provides for nesting of both connecting links with the ring components on the opposite sides. Thus, in the present invention, large numbers of connecting links can be included within a crimpable stent design.
Another aspect of the invention is to provide flexibility in the stent crimped on the catheter such that the stent can flex near the inflection points without significant radial expansion as the stent is subjected to bending along a longitudinal axis as it is advanced through bends in a coronary artery. As stents are advanced through tortuosities of a vessel, they are subjected to bending forces which can produce longitudinal stresses on the connector links. If the movement of connector links pulls the undulations open from their crimped position, the stent can become radially enlarged and have difficulty in crossing a narrow lesion. The present invention reduces the potential for this problem by aligning the connection of the links with the rings at a short, circumferentially extending portion, by providing curvature in the links which are then able to flex and thereby reduce stress on the junctions between the rings and the links and by providing xe2x80x9cdead endxe2x80x9d connections with the links which then avoids the transmission of forces from ring to ring throughout the length of the stent.
Yet another aspect of the invention is in connection with the stent configuration in which the undulating peaks and valleys of the rings are oriented such that the rings have peaks and valleys which are paired with each other in an in-phase relationship. In such a configuration, a link can be provided which interconnects with the rings at points on the rings which are substantially centered between the respective peaks and valleys of the rings and yet allows the link to nest within the peaks and valleys of the rings. This can be accomplished by providing at least two curved segments in the link in a central portion of the link.
Yet another aspect of the invention is in connection with the stent configuration in which the undulating peaks and valleys of the first ring are arranged with the undulating peaks and valleys of the second ring such that the first and second rings appear as mirror images of each other. In such a configuration, a link can be provided which interconnects with the rings at points on the rings which are substantially centered between the respective peaks and valleys of the rings and yet allows the link to nest within the peaks and valleys of the rings. This can be accomplished by providing a link including a sharp radius segment at each end and a gently curved central segment.